Polymeric compositions are used extensively as primary insulation materials for wire and cable. As an insulator, it is important that the composition have various physical and electrical properties, such as resistance to mechanical cut through; stress crack resistance; and dielectric failure. Unfortunately, the efficient use of polymeric compositions in high voltage cables has been hampered by a degradation process called “treeing.”
Treeing is a relatively slow progressive degradation of an insulation caused by electron and ion bombardment of the insulation resulting in the formation of microchannels or tubes having a tree-like appearance, hence the name. A tree initiates at points of contamination or voids that are foreign to the polymeric insulation by the action of ionization (corona) during high voltage surges. Once a tree starts it usually grows, particularly during further high voltage surges, and at some undetermined time, dielectric failure can occur.
There are two types of treeing: (1) electrical treeing and (2) water treeing. Water or electrochemical trees form in the presence of water and in particular at low voltages. When water is absent, the trees that form are called electrical trees.
Electrical treeing results from internal electrical discharges that decompose the dielectric. High voltage impulses can produce electrical trees. The damage that results from the application of alternating current voltages to the electrode/insulation interfaces, which can contain imperfections, is commercially significant. In this case, very high, localized stress gradients can exist and with sufficient time can lead to initiation and growth of trees
A common practice used to reduce the possibility of tree generation is to introduce additives into the polymeric compositions, which are often referred to as “voltage stabilizers.” Additives function in a variety of ways: (1) to capture energetic electrons chemically; (2) to slow down discharge path growth electrically; (3) to make the surfaces of internal cavities conductive; (4) to increase the bulk conductance to grade the field; and (5) to interfere physically with tree propagation. Gases, oils, liquids, waxes antioxidants, catalyst stabilizers, and mineral fillers of low hygroscopicity are all candidates for compounding agents for this purpose.
Voltage stabilizers, such as acetophenone, fluoranthene, pyrene, naphthalene, o-terphenyl, vinylnaphthalene, chrysene, anthracene, alkylfluoranthenes and alkylpyrenes, are thought to trap and deactivate electrons, and thus inhibit treeing. However, the volatility, migration, low solubility, and toxicity of the voltage stabilizers have limited their commercial success. When the volatility of the compound is too great, the compound will migrate to the surface, and evaporate, thereby eliminating the effectiveness of the compound. In addition, the compounds are toxic, and thus migration of the compounds to undesired locations, is problematic.
Silicones have found limited use in the area of anti-treeing. U.S. Pat. No. 3,956,420 discloses the use of a combination of ferrocene, in 8-substituted quinoline, and a silicone liquid to increase the dielectric strength of polyethylene and its voltage endurance in water. U.S. Pat. No. 4,144,202 inhibits water treeing in ethylene polymer compositions by employing organosilanes containing an epoxy radical. U.S. Pat. No. 4,263,158 further discloses the use of organosilanes containing carbon-nitrogen double bonds to inhibit water treeing in ethylene polymers.
Water tree growth and electrical tree growth in primary insulation still remains an important problem as treeing is still associated with dielectric failure. Thus, a need still exists for voltage stabilizers with low toxicity, low volatility and good compatibility with polyolefins, which can inhibit or retard treeing.